CN115360322A - A positive electrode sheet and lithium secondary battery - Google Patents

Abstract

In order to overcome the problem of low passing rate of lithium-ion batteries in acupuncture tests in the prior art, the application provides a positive electrode sheet and a lithium secondary battery. The positive electrode sheet includes a current collector, a first active material layer, and a second active material layer. The first active material layer is arranged on the surface of the current collector, the second active material layer is arranged on the surface of the first active material layer away from the current collector, and the membrane conductivity of the first active material layer is Rate satisfies the following relationship: 0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.01S/cm, among which, 0.01≤β≤0.1, 80%≤W ₁ ≤90%, 0.005S /cm≤δ ₁ ≤1S/cm, 1× ^10-5 S/cm≤δ ₂ ≤5× ^10-5 S/cm, 5%≤W ₂ ≤20%; a positive electrode sheet provided by this application can Improve the membrane resistance of the positive electrode sheet, reduce the short-circuit current during needle punching, thereby improving the battery needle punching pass rate and safety performance.

Description

A positive electrode sheet and lithium secondary battery

technical field

The invention belongs to the technical field of lithium ion batteries, and in particular relates to a positive electrode sheet and a lithium secondary battery.

Background technique

Efficient and reliable energy storage systems are essential to our modern society. Lithium-ion batteries with excellent performance are widely used in portable electronic products and electric vehicles, but frequent fires and explosions limit their further and wider applications. Lithium-ion battery safety has become a key issue in the lithium-ion battery industry.

There have been many recent reports of fire-related accidents and breakdowns caused by lithium-ion batteries. Due to their high operating voltage, high energy density, and use of flammable organic electrolytes, LIBs are energy devices with ignition potential. Therefore, lithium-ion batteries designed for commercial use must pass safety standards and undergo safety tests, such as electrical tests, environmental tests, and mechanical tests. These safety standard tests are used to evaluate battery safety performance and provide guarantees for the safe use of batteries. The needle stick test is a widely used lithium-ion safety test to evaluate batteries for internal short circuits, one of the leading causes of battery fires. The passing rate of the acupuncture test for existing lithium batteries is relatively low, because when acupuncture is performed, a short-circuit current is generated inside the battery cell, which causes the temperature of the battery cell to rise rapidly, and subsequently causes thermal decomposition of the internal material of the battery cell to fail; and in the analysis of the short-circuit mode, it can be known , the temperature rises the highest when the aluminum foil and the negative electrode are short-circuited, which is the main failure point of thermal runaway.

Contents of the invention

Aiming at the problem of low passing rate of lithium-ion battery acupuncture test in the prior art, the present application provides a positive electrode sheet and a lithium secondary battery.

In order to solve the above technical problems, the present application provides a positive electrode sheet, including a current collector, a first active material layer and a second active material layer, the first active material layer is arranged on the surface of the current collector, and the second active material layer The material layer is disposed on the surface of the first active material layer away from the current collector, the first active material layer includes a first active material and a first resistance increasing agent, and the membrane conductivity of the first active material layer is rate satisfies the following relationship:

0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.01S/cm

Among them, 0.01≤β≤0.1, 80%≤W ₁ ≤90%, 0.005S/cm≤δ ₁ ≤1S/cm, 1×10 ^-5 S/cm≤δ ₂ ≤5×10 ^-5 S/cm, 5% _≤W2≤20 %;

β is the impact factor;

_δ1 is the powder conductivity of the first active material, in S/cm;

δ ₂ is the powder conductivity of the first resistance increasing agent, unit S/cm;

W ₁ is the mass proportion of the first active material in the first active material layer, in %;

W ₂ is the mass proportion of the first resistance increasing agent in the first active material layer, in %.

Preferably, the conductivity of the membrane of the first active material layer satisfies the following relationship:

0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.005S/cm.

Preferably, the influence factor β is 0.01S/cm-0.05S/cm.

Preferably, the first active material includes lithium iron phosphate and its derivatives;

The powder conductivity _δ1 of the first active material is 0.004S/cm～1S/cm;

The mass proportion _W1 of the first active material in the first active material layer is 85%-90%.

Preferably, the first resistance increasing agent comprises a ceramic material;

The mass proportion W ₂ of the first resistance increasing agent in the first active material layer is 5%˜15%.

Preferably, the first active material layer further includes a first binder and a first conductive agent, and the mass proportion of the first binder in the first active material layer is 2.0% to 10.0%; The mass proportion of the first conductive agent in the first active material layer is 0%˜1%.

Preferably, the second active material layer includes a second active material, a second binder and a second conductive agent;

Based on the weight of the second active material layer being 100%, the mass proportions of the second active material, the second binder, and the second conductive agent are (94-97): (1-2): (2 ~5).

Preferably, the positive electrode sheet further includes an insulating layer, the insulating layer is disposed on the current collector and is disposed side by side with the first active material layer, and the insulating layer includes a ceramic material.

Preferably, the thickness of the insulating layer is 10um-30um.

In another aspect, the present application provides a lithium secondary battery, including a negative electrode sheet, a separator, and the above-mentioned positive electrode sheet.

Beneficial effect:

Compared with the prior art, the positive electrode sheet of the present application is coated with the first active material layer and the first active material layer on the surface of the current collector, and the conductivity of the first active material layer is required to satisfy the relational expression 0.001S/cm≤ β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.01S/cm, which can increase the sheet resistance of the positive electrode sheet and reduce the short-circuit current during acupuncture, thereby improving the battery acupuncture pass rate and safety performance; at the same time The first active material layer also cooperates with the second active material layer to ensure that the electrical performance of the lithium-ion battery is not lost.

Description of drawings

Fig. 1 is the structural representation of positive plate;

1. Current collector; 2. First active material layer; 3. Second active material layer; 4. Insulating layer.

Detailed ways

In order to make the technical problems, technical solutions and beneficial effects solved by the present invention clearer, the present invention will be further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

A positive electrode sheet provided by the present application includes a current collector 1, a first active material layer 2 and a second active material layer 3, the first active material layer 2 is arranged on the surface of the current collector 1, and the second active material layer The material layer 3 is disposed on the surface of the first active material layer 2 away from the current collector 1, the first active material layer 2 includes a first active material and a first resistance increasing agent, and the first active material layer The conductivity of the membrane of 2 satisfies the following relationship:

0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.01S/cm

Among them, 0.01≤β≤0.1, 80%≤W ₁ ≤90%, 0.005S/cm≤δ ₁ ≤1S/cm, 1×10 ^-5 S/cm≤δ ₂ ≤5×10 ^-5 S/cm, 5% _≤W2≤20 %;

β is the impact factor;

_δ1 is the powder conductivity of the first active material, in S/cm;

δ ₂ is the powder conductivity of the first resistance increasing agent, unit S/cm;

W ₁ mass proportion of the first active material in the first active material layer 2, unit %;

W ₂ is the mass proportion of the first resistance increasing agent in the first active material layer 2 , in %.

The positive electrode sheet of the present application is coated with a first active material layer 2 and a second active material layer 3 on the surface of the current collector 1, wherein a first resistance increasing agent is added to the first active material layer 2, and the first active material layer is required 2, the conductivity of the diaphragm satisfies the relational formula 0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.01S/cm, which increases the diaphragm resistance of the positive electrode sheet of the lithium-ion battery and reduces the needle Short-circuit current during stabbing, thereby improving the pass rate and safety performance of battery acupuncture. The first active material layer 2 provided on the positive electrode sheet not only has the function of increasing the resistance of the positive electrode sheet and reducing the short-circuit current during acupuncture, but also cooperates with the second active material layer 3 to ensure that the electrical performance of the lithium-ion battery is not lost.

In some preferred embodiments, the conductivity of the membrane of the first active material layer 2 satisfies the following relationship:

0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.005S/cm.

Specifically, when the membrane conductivity of the first active material layer 2 satisfies the relational expression 0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.005S/cm, the membrane resistance of the positive electrode sheet Higher, the pass rate is higher when the battery is subjected to the needle test, which is more conducive to reducing the short-circuit current when the battery is needled, and improving the safety performance of the battery.

The membrane conductivity of the first active material layer 2 is calculated according to the relational formula β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ ), and its value can be 0.001S/cm, 0.003S/cm, 0.005S/cm , 0.007S/cm, 0.009S/cm, 0.01S/cm, 0.002S/cm, 0.004S/cm, 0.006S/cm, 0.008S/cm, etc., as long as the membrane conductivity of the first active material layer 2 satisfies The relational expression 0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.01S/cm is sufficient.

In some embodiments, β is an impact factor, and the range of β is 0.01≤β≤0.1.

Specifically, for example, β can be 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 and so on.

The larger the mass ratio of the first conductive agent in the first active material layer 2, the higher the electrical conductivity of the prepared first active material layer 2 diaphragm, and the higher the short-circuit current of the prepared battery when needled; the first The smaller the mass proportion of the conductive agent, the lower the conductivity of the membrane of the first active material layer 2, and the lower the short-circuit current when the battery is acupunctured. After a lot of research, the inventor found that controlling the conductivity and mass proportion of the first active material, adding a first resistance increasing agent, and controlling the mass proportion of the first conductive agent can increase the sheet resistance of the first active material layer 2 . The inventors found that the value of β is related to the mass proportion of the first binder in the first active material layer 2, the larger the mass proportion of the first binder, the smaller the value of β; the first conductive agent The larger the mass ratio, the larger the β value. After a lot of research, the inventor found that, if β is in the range of 0.01 to 0.1, the conductivity and mass ratio of the first active material and the first resistance increasing agent are limited, and the conductivity of the prepared first active material layer 2 satisfies the relationship The formula 0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.01S/cm can increase the sheet resistance of the lithium-ion battery, reduce the short-circuit current when the battery is needled, and improve the safety of the battery performance.

In some preferred embodiments, the impact factor β is 0.01-0.05.

In some embodiments, the powder conductivity δ ₁ of the first active material is 0.005S/cm≤δ ₁ ≤1S/cm; the mass of the first active material in the first active material layer 2 accounts for The ratio W ₁ is 80%≤W _1≤90 %.

Specifically, the powder conductivity of the first active material is in the range of 0.005 S/cm to 1 S/cm, which helps to increase the sheet resistance of the first active material layer 2; for example, the powder conductivity of the first active material can be 0.005 S/cm, 0.01S/cm, 0.04S/cm, 0.06S/cm, 0.08S/cm, 0.1S/cm, 0.3S/cm, 0.5S/cm, 0.7S/cm, 0.8S/cm, 0.9 S/cm, 1.0S/cm, etc. The mass proportion W ₁ of the first active material in the first active material layer 2 is 80%≦W ₁ ≦90%, which ensures that the capacity and other electrical properties of the battery will not be lost.

If the powder conductivity _δ1 of the first active material is greater than 1 S/cm, the sheet resistance of the prepared first active material layer 2 will be lower, and the penetration rate of the battery will decrease. If the mass ratio W of the first active material layer ₂ is greater than 90%, although the capacity of the battery can be increased correspondingly, the sheet resistance of the first active material layer 2 does not satisfy the relational expression 0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.01S/cm, the sheet resistance of the first active material layer 2 is reduced, the conductivity of the positive electrode sheet is high, and the short-circuit current of the prepared battery is relatively high during acupuncture.

In some preferred embodiments, the first active material includes lithium iron phosphate and its derivatives; the mass ratio W ₁ of the first active material in the first active material layer 2 is 85% to 90%. %; the powder conductivity δ ₁ of the first active material is 0.004S/cm～1S/cm.

In some embodiments, the mass ratio W of the first resistance increasing agent in the first active material layer 2 _is 5% to 20%; the powder conductivity _δ of the first resistance increasing agent is 1×10 ^-5 S/cm to 5×10 ^-5 S/cm.

Specifically, the mass proportion of the first active material layer 2 is 5% to 20%, and the first resistance increasing agent whose conductivity is in the range of 1×10 ^-5 S/cm to 5×10 ^-5 S/cm , to reduce the conductivity of the first active material layer 2, so as to achieve the purpose of reducing the short-circuit current when the battery is needle-punched, and improve the needle-puncture pass rate of the battery. The mass proportion of the first resistance increasing agent may be 5%, 8%, 10%, 12%, 14%, 15%, 16%, 18%, 20% and so on.

If the mass proportion of the first resistance increasing agent is less than 5%, the sheet resistance of the first active material layer 2 is reduced; if the mass proportion of the first resistance increasing agent is greater than 20%, the content of the first living active material is reduced, and the battery The capacity of the battery is reduced and the electrical performance of the battery is lost. If the conductivity of the first resistance increasing agent is higher than 5×10 ^-5 S/cm, the conductivity value of the membrane of the first active material layer 2 is relatively high, and the conductivity of the membrane of the first active material layer 2 cannot be reduced. Effect.

In some preferred embodiments, the first resistance increasing agent includes a ceramic material; the mass ratio W ₂ of the first resistance increasing agent in the first active material layer 2 is 5%-15%.

Specifically, ceramic materials such as alumina, silicon oxide, silicon nitride, zirconia, silicon carbide, boron nitride, titanium oxide, magnesium oxide, beryllium oxide, aluminum carbide materials, silicon carbide alloy materials, zirconium carbide, One or more of titanium carbide and titanium nitride.

In some embodiments, the first active material layer 2 further includes a first binder and a first conductive agent, and the mass ratio of the first binder in the first active material layer 2 is 2.0 %˜10.0%; the mass proportion of the first conductive agent in the first active material layer 2 is 0%˜1%.

Specifically, the first binder includes one or more of styrene-butadiene rubber, water-based acrylic resin, carboxymethyl cellulose, polyvinylidene fluoride, ethylene-vinyl acetate copolymer, and polyvinyl alcohol. The first conductive agent includes one or more of graphite, carbon black, graphene, carbon nanotubes, carbon nanofibers, SuperP, and acetylene black.

The mass ratio of the first binder in the first active material layer 2 is between 2% and 10%, and the content of the first binder is relatively high, which increases the adhesion between the first active material layer 2 and the current collector 1. The bonding strength prevents the first active material layer 2 from detaching from the current collector 1 when the battery is needle-punched, and reduces the risk of contact between the aluminum foil and the negative electrode sheet.

In some embodiments, the second active material layer 3 includes a second active material, a second binder, and a second conductive agent; based on the mass of the second active material layer 3 as 100%, the second The mass ratio of the active material, the second binder, and the second conductive agent is (94-97): (1-2): (2-5).

The second active material includes one or more of lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, lithium iron phosphate, and lithium vanadium phosphate. The second binder includes one or more of styrene-butadiene rubber, water-based acrylic resin, carboxymethyl cellulose, polyvinylidene fluoride, ethylene-vinyl acetate copolymer, and polyvinyl alcohol. The second conductive agent includes one or more of graphite, carbon black, graphene, carbon nanotube, carbon nanofiber, SuperP, and acetylene black.

In the second active material layer, the proportion of the second active material is between 94% and 97%, the content of the second active material is high, and the content of lithium ions is high, which ensures the capacity of the battery. The mass proportion of the second conductive agent is between 2% and 5%, which improves the conductivity of the second active material layer 3, is beneficial to the electrochemical reaction of the battery, and improves the electrical performance of the battery.

In some embodiments, the positive electrode sheet further includes an insulating layer 4, the insulating layer 4 is disposed on the current collector 1 and arranged side by side with the first active material layer 2, and the insulating layer 4 includes ceramic Material.

Specifically, the insulating layer 4 is coated on the empty foil to avoid the risk of battery short circuit caused by the contact between the aluminum foil and the negative electrode sheet during the battery needle test, and improve the safety performance of the lithium-ion battery. It should be noted that if the number of empty foil locations is N, the number of insulating layers 4 coated on the empty foil locations is less than or equal to N. As shown in FIG. 1, an insulating layer 4 is applied to both empty foils.

In some embodiments, the thickness of the insulating layer 4 is 10um˜30um.

Specifically, the insulating layer 4 plays the role of preventing the negative electrode sheet from contacting the aluminum foil when the battery is needled. If the thickness of the insulating layer 4 is less than 10mm, the battery is easy to pierce the insulating layer 4 when the battery is needled, and the aluminum foil contacts the negative electrode sheet, causing battery damage. Risk of short circuit. If the thickness of the insulating layer 4 is higher than 30 mm, the thickness of the battery cell is increased to reduce the energy density of the battery.

The positive electrode current collector 1 is selected from metal materials that can conduct electrons. Preferably, the positive electrode current collector 1 includes one or more of Al, Ni, tin, copper, and stainless steel. In a more preferred embodiment, the The positive current collector 1 is selected from aluminum foil.

On the other hand, the present application provides a lithium secondary battery, including a negative electrode sheet, a separator, and the above-mentioned positive electrode sheet.

The negative electrode sheet is obtained by an existing preparation method, which will not be repeated here.

The lithium secondary battery provided by the application uses the above-mentioned positive electrode sheet, the sheet resistance of the positive electrode sheet is increased, the short-circuit current when the battery is needled is reduced, and the battery needle can be effectively improved without reducing the electrical performance of the battery. Thorn pass rate and safety performance.

The specific embodiments of the present invention will be further explained through the following examples, but it does not mean that the protection scope of the present invention is limited to the scope described in the examples.

Example

This embodiment is used to illustrate the lithium secondary battery disclosed in the present application.

Prepare lithium secondary battery, comprises the steps:

Prepare the positive electrode sheet:

Preparation of the first positive electrode active material layer slurry:

The first active material (W ₁ is 80% ~ 90%, δ ₁ is 0.025S/cm ~ 1S/cm lithium iron phosphate), the first conductive agent (0wt% ~ 1wt% conductive carbon black), the first resistor Increaser (W ₂ is 5% to 20%, δ ₂ is 1×10 ^-5 S/cm to 5×10 ^{- 5} S/cm alumina ceramic particles) and the first binder (2 to 10wt% poly Vinyl fluoride) were mixed, and then N-methylpyrrolidone was added to stir and disperse to form the slurry of the first positive electrode active material layer.

Preparation of the second positive electrode active material layer slurry:

Mix the second positive electrode active material (94-97wt% lithium cobaltate), the second conductive agent (2-5%wt% conductive carbon black) and the second binder (1-2wt% polyvinylidene fluoride), and then Add N-methylpyrrolidone and stir to disperse to form a slurry of the first positive electrode active material layer.

Preparation of insulating layer 4 slurry:

Stir and disperse (70-90) wt% alumina ceramic material, (10-30) wt% polyvinylidene fluoride or polybutadiene acrylonitrile, and N-methylpyrrolidone to form a slurry for the insulating layer 4 .

Then the first positive electrode active material layer slurry is coated on the surface of the aluminum foil by coating equipment, and the second positive electrode active material layer slurry is coated on the surface of the first active material layer 2 away from the aluminum foil; The material is coated on the empty foil part of the aluminum foil to form a slurry for the insulating layer 4; after drying, rolling, cutting, and preparing a positive electrode sheet. The thickness of the first active material layer 2 is 5-20 μm, the thickness of the second active material layer 3 is 60-90 μm, and the thickness of the insulating layer 4 is 10-30 μm.

Preparation of negative electrode sheet:

According to the mass ratio of 94:1:2.5:2.5, take negative electrode active material hard carbon, conductive carbon black, binder styrene-butadiene rubber and carboxymethyl cellulose to mix, and then disperse them in an appropriate amount of deionized water to obtain Negative electrode slurry: coating the slurry on both sides of the copper foil, drying, rolling, cutting, and preparing the negative electrode sheet, the thickness of the negative electrode sheet is between 70-120 μm.

Preparation of lithium secondary battery:

The above-mentioned positive electrode sheet, negative electrode sheet, separator, and aluminum-plastic film are made into a battery, and then liquid injection, chemical formation and other processes are performed, and finally the electrical performance test and acupuncture test are performed on the battery.

Embodiment 1-10 and comparative example 1-10

Examples 1-10 and Comparative Examples 1-10 prepared batteries according to the above preparation method, wherein the mass content W ₁ and conductivity δ ₁ of the first active material, the mass content W ₂ and conductivity δ ₂ of the first resistance increasing agent , The specific data of impact factor β value are shown in Table 1.

Example 11

The difference between Example 11 and Example 1 is that Example 11 does not coat the insulating layer 4 on the empty foil area of the current collector 1 .

Comparative example 11

The difference between Comparative Example 11 and Example 1 is that in Comparative Example 1, two layers of slurry for the second active material layer 3 are coated on the surface of the current collector 1 , and the rest is the same as that of Example 1.

Table 1 Example 1-11 and Comparative Example 1-11 The first active material layer 2 data table

Battery performance test:

(1) Battery internal resistance and energy density: At room temperature, the battery 1C prepared in Examples 1-11 and Comparative Examples 1-11 was charged to 4.2V at a constant current, and then charged at a constant voltage of 4.3V, with a cut-off current of 0.05C , to test the internal resistance of the battery. Afterwards, discharge the battery at a constant current of 1C to 3.0V, test the discharge capacity C1 of the battery, and calculate the energy density of the battery; see Table 2 for specific data.

(2) Cycle test at room temperature at 25°C:

The batteries prepared in Examples 1-11 and Comparative Examples 1-11 were charged to 4.2V with a constant current of 1C at a normal temperature of 25°C, and then charged at a constant voltage of 4.2V with a cut-off current of 0.05C, and then charged at a constant current of 1C. Current discharge to 3.0V, so cycle 500 cycles;

Calculate the 500-cycle capacity retention rate=the discharge capacity at the 500th cycle/the average value of the cycle discharge capacity at the 1st-3rd cycle×100%.

(3) Acupuncture test:

The batteries prepared in Examples 1-11 and Comparative Examples 1-11 were subjected to acupuncture tests. The test process was as follows: use a high-temperature-resistant steel needle with a diameter of 4 ± 0.5 mm at a speed of (30 mm/s ± 5 mm/s) , from the direction perpendicular to the cell plate, the puncture position should be close to the geometric center of the punctured surface (the steel needle stays in the cell). After 1 hour of acupuncture, observe the acupuncture passage of the cell, and stop the experiment.

The electrical performance test data are shown in Table 2.

Table 2 Example 1-11 and comparative example 1-11 battery performance test data table

It can be seen from Table 1-2 that there is no first active material layer 2 in Comparative Example 11, and although the normal temperature cycle capacity retention rate of the battery is higher than that of Example 1, the penetration rate of the battery is 0; The surface is coated with the first active material layer 2, and the membrane conductivity relation of the first active material layer 2 is 0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.01S/cm high When it is greater than 0.01, the acupuncture pass rate of the battery is increased to 2. Compared with Comparative Example 11, the resistance in Comparative Example 2 increases. It is guessed that increasing the conductivity of the positive electrode sheet can increase the internal resistance of the battery and improve the acupuncture pass rate of the battery. rate; the relational formula of the membrane conductivity of the first active material layer 2 in Examples 1-9 0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.01S/cm value 0.001-0.01 Among them, the acupuncture penetration rate of the battery reaches 100%, and the value of the membrane conductivity relational expression of the first active material layer 2 in Comparative Example 1 is lower than 0.001, and the cycle performance of the battery is reduced; The diaphragm resistivity of the first active material layer 2 is between 0.001 and 0.01. It is guessed that during the acupuncture process of the battery, when the current passes through the first active material layer 2, the pole piece has a relatively large resistance, which prevents the battery from rapidly generating heat, Combustion, etc., thereby improving the needle penetration rate of the battery.

Comparing Comparative Examples 3 and 4 with Example 9, the first active material W ₁ in Comparative Example 3 is small, and the energy density of the battery is reduced; in Comparative Example 4, W ₁ is large, and the acupuncture pass rate of the battery is reduced, and it is guessed that W1 is relatively large , the binder content of the battery is low, and the binding force between the first active material layer 2 and the current collector 1 is reduced, thereby reducing the penetration rate of the battery through acupuncture. Compared with Example 10 and Comparative Example 5, the electrical conductivity _δ1 of the first active material is higher than 1S/cm, and the acupuncture pass rate is reduced. It is guessed that _δ1 is higher than 1S/cm, and the flow resistance of the current when the battery is acupunctured decreases; In Comparative Example 6, the conductivity δ ₁ of the first active material is lower than 0.005 S/cm, and the cycle performance of the battery is reduced. It is guessed that δ ₁ is lower than 0.005 S/cm, and the internal resistance of the battery increases, which reduces the cycle performance of the battery. Comparative Examples 7 and 8 are compared with Examples 1-10, W _is higher than 20% or lower than 5%, and the normal temperature cycle capacity retention rate of the battery is lower than that of Examples, indicating that the content _W of the first resistance increasing agent affects the battery. cycle performance. Comparative examples 9 and 10 are compared with Examples 1-10, the first resistance increasing agent δ ₁ is lower than 0.00001S/cm, which affects the cycle performance of the battery; the first resistance increasing agent δ ₁ is higher than 0.00005S/cm, which reduces battery acupuncture Passing rate. Comparing Examples 1-10 with Example 11, the empty foil area of the current collector 1 in Example 11 is not coated with the insulating layer 4, and the throughput rate of the battery is reduced.

Through the comparison of Examples 1-10 and Comparative Examples 1-11, it can be concluded that the membrane conductivity of the first active material layer 2 coated on the surface of the current collector 1 satisfies the relational formula 0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.01S/cm, and 0.01≤β≤0.1, 80%≤W ₁ ≤90%, 0.005S/cm≤δ ₁ ≤1S/cm, 1×10 ^-5 S/cm≤ δ ₂ ≤5×10 ^-5 S/cm, 5% ≤ W ₂ ≤ 20%; it can increase the sheet resistance of the positive electrode sheet and reduce the short-circuit current during acupuncture, thereby improving the battery acupuncture pass rate and safety performance; at the same time The first active material layer 2 also cooperates with the second active material layer 3 to ensure that the electrical performance of the lithium-ion battery is not lost.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present invention should be included in the protection of the present invention. within range.

Claims (10)

1. A positive electrode sheet, characterized in that it comprises a current collector, a first active material layer and a second active material layer, the first active material layer is arranged on the surface of the current collector, and the second active material layer is arranged On the surface of the first active material layer away from the current collector, the first active material layer includes a first active material and a first resistance increasing agent, and the membrane conductivity of the first active material layer satisfies the following Relational formula: 0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.01S/cm Among them, 0.01≤β≤0.1, 80%≤W ₁ ≤90%, 0.005S/cm≤δ ₁ ≤1S/cm, 1×10 ^-5 S/cm≤δ ₂ ≤5×10 ^-5 S/cm, 5% _≤W2≤20 %; β is the impact factor; _δ1 is the powder conductivity of the first active material, in S/cm; δ ₂ is the powder conductivity of the first resistance increasing agent, unit S/cm; W ₁ is the mass proportion of the first active material in the first active material layer, in %; W ₂ is the mass proportion of the first resistance increasing agent in the first active material layer, in %. 2. The positive electrode sheet according to claim 1, wherein the conductivity of the diaphragm of the first active material layer satisfies the following relationship: 0.001S/cm≤β×(δ ₁ ×W ₁ +δ ₂ ×W ₂ )≤0.005S/cm. 3 . The positive electrode sheet according to claim 1 , wherein the influence factor β is 0.01˜0.05. 4. The positive electrode sheet according to claim 1, wherein the first active material comprises lithium iron phosphate and derivatives thereof; The powder conductivity _δ1 of the first active material is 0.004S/cm～1S/cm; The mass proportion _W1 of the first active material in the first active material layer is 85%-90%. 5. The positive electrode sheet according to claim 1, wherein the first resistance increasing agent comprises a ceramic material; The mass proportion W ₂ of the first resistance increasing agent in the first active material layer is 5%˜15%. 6. The positive electrode sheet according to claim 1, wherein the first active material layer further comprises a first binder and a first conductive agent, and the first binder is placed on the first active material layer. The mass proportion of the layer is 2.0%-10.0%; the mass proportion of the first conductive agent in the first active material layer is 0%-1%. 7. The positive electrode sheet according to claim 1, wherein the second active material layer comprises a second active material, a second binder and a second conductive agent; Based on the weight of the second active material layer being 100%, the mass proportions of the second active material, the second binder, and the second conductive agent are (94-97): (1-2): (2 ~5). 8. The positive electrode sheet according to claim 1, wherein the positive electrode sheet further comprises an insulating layer, the insulating layer is arranged on the current collector, and is arranged side by side with the first active material layer, so The insulating layer includes a ceramic material. 9 . The positive electrode sheet according to claim 8 , wherein the insulating layer has a thickness of 10um˜30um. 10. A lithium secondary battery, characterized in that it comprises a negative electrode sheet, a separator, and the positive electrode sheet according to any one of claims 1-9.

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